Edit July 3, 2014:
Originally written based on Android 4.0, this post is now over two years old. I now own a Nexus 5 running Android 4.4.4, and the battery monitoring screen has apparently improved a great deal since version 4.0:
If any app holds a wake lock for an excessive amount of time, it will show up clearly as a power hog on the Settings –> Battery screen. This should make battery life troubleshooting much easier than it was in the past. Kudos to Google for this improvement.
Here is the original post from Feb 16, 2012:
If you have an Android device and enjoy installing and using many different apps, chances are that you may have run into the problem of rapid battery drain even while the screen is off.
If you Google “how to save battery on Android”, you will often find tips such as disabling various wireless radios, lowering your screen timeout, lowering your brightness, etc. These tips may help, but a potentially more serious culprit is some third-party app constantly keeping the system awake, draining power in the background. If this is the case on your phone, it will by far outweigh the consumption of your wireless radios, possibly reducing your battery life to as little as 8 hours.